import tensorflow as tf

tf.compat.v1.disable_eager_execution()  # ATTENTION

x = tf.compat.v1.placeholder(name='x', shape=[], dtype=tf.string)
y = tf.compat.v1.placeholder(name='y', shape=[], dtype=tf.string)
z = tf.strings.join([x, y], name="join", separator=" ")

with tf.compat.v1.Session() as sess:
    zv = sess.run(fetches=z, feed_dict={x: 'Hello', y: 'world'})
    print(zv)
